Re: [PATCH v2 3/4] scsi_dh_alua: do not call BUG_ON when updating port group

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Mon, 2017-05-15 at 10:16 +0200, Martin Wilck wrote:
> On Fri, 2017-05-12 at 16:24 +0000, Bart Van Assche wrote:
> > Allowing races like the one this patch tries to address to exist
> > makes the ALUA code harder to maintain than necessary. Have you
> > considered to make alua_bus_detach() wait until ALUA work has
> > finished by using e.g. cancel_work_sync() or rcu_synchronize()?
> 
> to be honest, no, I didn't consider this yet. The current kernel
> crashes with BUG() if an ALUA device is detached at an inopportune
> point in time (not just theoretically, we actually observed this). The
> goal of my patch was to fix this with minimum risk to introduce other
> problems. The addition in patch 4/4 was an attempt to address the
> concern you had expressed in your review of the v1 patch.
> 
> I'm not opposed to try to find a better solution, but could we maybe
> get the fix for the BUG() (i.e. patch 3/4) applied in the first place?
> AFAICS it would not conflict with a solution like the one you
> suggested.

Hello Martin,

Sorry but I don't think it's a good idea to merge patch 3/4 in the upstream
kernel. Even with that patch applied there is nothing that prevents that
h->handler_data would be freed while alua_rtpg() is in progress and hence
that h->sdev is a completely random pointer if alua_rtpg() is executed
concurrently with alua_bus_detach(). Please do not try to paper over race
conditions but fix these properly.

Bart.